A flash memory that is mainstream at present and that uses a floating gate, has a problem that as the size of a memory cell is reduced, variations in threshold voltage (Vth) occur due to interference by capacitive coupling between the floating gates of adjacent cells. Hence, as a memory suitable for the reduction in size, a variable resistance nonvolatile storage element with a variable resistance layer sandwiched between electrodes is being developed. This variable resistance nonvolatile storage element is characterized in that the electrical resistance of the resistance layer is electrically stimulated so as to be switched between two values or more; since the variable resistance nonvolatile storage element is simple in structure and operation, it is expected to be a nonvolatile storage element that can be reduced in size and cost.
As a layer (a variable resistance layer) in which resistance varies with an applied voltage, there are oxides of elements selected from a group formed of transition metals; examples of the oxides include a nickel oxide (NiO), a vanadium oxide (V2O5), a zinc oxide (ZnO), a niobium oxide (Nb2O5), a titanium oxide (TiO2), a tungsten oxide (WO3), a cobalt oxide (CoO) and a tantalum oxide (Ta2O5). As the variable resistance layer, there are metallic oxides that have compositions displaced from stoichiometric compositions formed of Ni, Ti, Ta, Hf, Nb, Zn, W, Co and the like.
Although the principle of operation of the resistance change is unclear, the following principles have been reported: a voltage is applied to the variable resistance layer, thus a current path called a filament is formed in the variable resistance layer and the resistance of the element varies with the state of the connection between the filament and upper and lower electrodes; the resistance of the variable resistance layer varies by the movement of oxygen atoms in the interface between the electrodes and the variable resistance layer.
FIG. 12 is a schematic diagram showing the cross-sectional structure of a conventional variable resistance nonvolatile storage element (ReRAM: resistive random access memory) with reference to patent document 1. The variable resistance element (memory element) 610 of a general ReRAM has a parallel-plate type laminate structure in which a variable resistance film (for example, a transition metal oxide film) 613 is sandwiched between a lower electrode 612 formed on an interlayer insulation film 611 and an upper electrode 614. When a voltage is applied between the upper electrode 614 and the lower electrode 612, the electrical resistance of the variable resistance film 613 varies, and the variable resistance film 613 takes two different resistance states (the reset state and the set state). Patent document 2 discloses that, as the material of the upper electrode 614, a material containing Pt is used, and as the material of the lower electrode 612, a material containing at least one element selected from Ru, Ti, Al, Ta, Cu, W and Ni is used.
In the operating mechanism of the variable resistance element 610, as an initial operation for allowing the resistance state to be electrically transited between two resistance states, a forming voltage is first applied. The application of the forming voltage results in the state where a filament serving a current path can be formed in the variable resistance film 613. Thereafter, an operation voltage (a set voltage and a reset voltage) is applied, and thus the state of the generation of the filament is changed, with the result that a set/reset operation, that is, writing and deletion are performed. As the area of an operation region of the variable resistance element 610 is increased, the number of filaments is increased; when the number of filaments is increased, variations in the control of the reset current are produced, and consequently, an operation as a memory varies. The operation area of the variable resistance element 610 is preferably small so that the requirement for achieving high density is satisfied and a stable and highly reliable operation is realized. However, as described above, in the conventional structure, the size reduction is limited by the accuracy of processing in the photolithography technology.
Patent document 1 proposes a nonvolatile storage device in which Pt is used as upper and lower electrodes and a variable resistance layer is formed of NiO; patent document 1 discloses that a current path called filament is formed in a Ni oxide, and the resistance varies. Moreover, non-patent document 2 proposes a nonvolatile storage device in which Pt is used as upper and lower electrodes and a variable resistance layer is formed of TaOx; non-patent document 2 discloses that the resistance varies by the movement of oxygen atoms in the interface layer between the Pt electrode and TaOx.
Attention is given to a technology on a variable resistance element that uses as an electrode material a titanium nitride electrode on which etching processing is easily performed. Non-patent document 3 proposes a nonvolatile storage device in which Pt is used as a lower electrode, HfOx or HfAlOx is used as a variable resistance layer and TiN is used as an upper electrode; non-patent document 3 discloses that HfAlOx is used as the variable resistance layer and thus variations in operation voltage can be reduced. Furthermore, non-patent document 4 discloses that a TiN/Ti/HfO2/TiN laminate structure is subjected to oxygen annealing, thus a TiN/TiOx/HfOx/TiN laminate structure is produced and consequently, a resistance varying operation is achieved.    [Patent document 1] Japanese Unexamined Patent Application Publication No. 2009-141275    [Patent document 2] Japanese Patent No. 3919205    [Non-patent document 1] Applied Physics Letters 86, 093509 (2005)    [Non-patent document 2] International Electron Devices Meeting Technical Digest, 2008, p. 293 to p. 296    [Non-patent document 3] Symposium on VLSI Technology Digest of Technical Papers, 2009, p. 30 to p. 31    [Non-patent document 4] International Electron Devices Meeting Technical Digest, 2008, p. 297 to p. 300